crustal structure experiment beneath Wairarapa - Wellington area: results from SAHKE Tim Stern and SAHKE team* * VUW, GNS, University of Southern California, University of Tokyo(Japan) SAHKE = Seismic Array on the HiKurangi margin Experiment
Talk Plan Plate boundaries & earthquakes Little faults, Big faults and Megathrusts SAHKE and what goes on beneath the Wairarapa
Japanese Subduction Zone similar to what we see under eastern North Island
NZ earthquakes: the $ cost => $500 mill / y
Faults Hazards
Canterbury Earthquakes: New Fault?
Cook Strait Fault aftershocks, July 2013 = faults Epicenter of after-shock
Red: Crustal earthquakes M 2, 2000 to 2009 Blue: Shallow earthquakes 6 M < 7, 1930 to the present, no aftershocks. One every two years. Was the February 22, ChCh, M 6 earthquake particularly unusual?
NZ earthquakes: > Mag 7 Red highlight => associated with known active fault
Probabilistic Hazard in Japan => Most damaging eqs (since 1979) occur in low risk areas
Magnitude Problem for seismology in general Record is too short for prediction 9.6 9.4 9.2 9 8.8 8.6 8.4 8.2 8 Ecuador 1906 Great (M > 8) Earthquakes Since 1900 Chile 1960 Alaska 1964 Sumatra 2004 Russia 1952 Japan 2011 Alaska 1965 Chile 2010? 7.8 1900 1920 1940 1960 1980 2000 2020 Year Data: USGS
Science of Earthquakes Statistics After shocks with time Number of shocks with specific magnitudes Forecasts Physics *Geophysical exploration *Geology of fault zones. *Measurements of rupture rates
Pacific-Australia Plate Boundary Zone 50 mm/yr Australian Plate 40 mm/yr 30 mm/yr Pacific Plate
The SAHKE project Crust light and brittle Asthenospheric mantle -semi-fluid
GPS observations over 10 years Vectors showing velocity (mm/y) GPS => Major advance in our understanding of earthquakes
GPS ~ 10 y Wallace et al, 2004 The Wellington problem
MED spec seismic lines MED( MBIE) Exploration~ $30 m since 2005 Spec-seismic data
69000 offshore sound sources from 3200 km of ship profiles SAHKE I 2010 Jointly with Crown Minerals
SAHKE3 Multi-channel Subsidence since 4 Ma most since 2 Ma seismic data A A A A Processed by Geotrace, supplied courtesy of MED
0 60 km Uplift Wanganui Basin Coastal Plain Drowned topography and onlap in the Marlborough Sounds. Marlborough Sounds (subsidence) Wellington
Seismic Reflection data reveals hidden and buried Mountain Range, north of Marlborough Sounds Northern Alps => semi-alpine mtn range ~1800 m high ~ 8 km ~ 200 km SAHKE 2 ( west side)
~ 20 km 1800 m
SAHKEII: Seismic Array Hikurangi Experiment 2011 SAHKE Experiment 835 seismic stations deployed at 100 m spacing (277 3-component and 558 vertical sensors) ~ 1400 seismographs http://juliansrockandiceblog.blogspot.com/2011_05_01_archive.html
SAHKE II: Seismic Array Hikurangi Experiment 2011 SAHKE Experiment 835 seismic stations deployed at 100 m spacing (277 3-component and 558 vertical sensors) 12 x500 kg explosive sources Crew of > 60 people 12 months planning 3 weeks fieldwork
Henrys et al, 2013 Top panel Data from shots _ a sort of X-ray Bottom panel: model of structure and earthquake locations
Martinborough 2x vertical exag Henrys et al (G-cubed, 2013) Green rock = sediment rafted in and underplated
East Cape : Note : the same mound of sediment on top of plate Bassett et al. G^3, 2010
Tohoku EQ Mag. 9 at depth of 24 km. Oceanic crust ( 2900 kg/m 3 ) = 7km thick
Plate interface ~ 24 km deep under Kapiti Kapiti Oceanic crust ~ 12 km thick - buoyant Wairarapa Crust Mantle Mantle
Difference between Japan and NZ : a proposal
What does the WW Fault do under the land surface? Tauherenikau River: Geophysics Field trip for VUW students
West Wairarapa fault is steep at surface then shallows at depth: => large Mag events for rupture length
Martinborough 2x vertical exag Henrys et al (G-cubed, 2013) Green rock = sediment rafted in and underplated
Isostatic Gravity Free-air gravity (offshore) Process for a broad, coeval, subsidence & uplift?
Whanganui Wairarapa 100-200 km Outer High: In Topography and gravity Weight of Subducted slab
Horizontal stress from collision Enhanced Outer High: In Topography and gravity Weight of Slab
Coast Aorangi Mts Numerical modelling : Apply principles of mechanical engineering to the earth In-plane stress Max upwarp ~ 1000 m (east Wairarapa). Flexure of plate with T e = 30 km And in plane-stress = 170 MPa
Wellington Wairarapa setting Land shot 80 km Image the base of the plate with seismic reflections- 100 km depth
Summary Subduction is very shallow under the Wairarapa SAHKE => much better understanding of Wairarapa Faults * Stress release valves from the mega-thrust? ( mag 7-8 events) * Subsidence in offshore Kapiti and uplift in Wairarapa appear to be mechanically linked
Shot 11 P1 P2 13-18 Hz
Blow out from 1.4 tonne shot In 50 m deep holes cased with 8 inch diameter steel
West Wairarapa Fault Martinborough ~10 km ~25 km Henrys et al., 2013
Re-Enhanced Outer High: In Topography and gravity Weight of Slab Horizontal stress from collision torque